It is increasingly common for foodstuffs and drinks to be stored in containers constructed of a synthetic material. A thermoplastic material such as polyethylene terephthalate (PET) is commonly employed for the synthetic material. In this context, plates or films of the thermoplastic synthetic material are first heated and then, in a subsequent work step, are pressed or otherwise formed into a specific, desired shape using, for example, a press defining a corresponding contour, compressed air, or by generating a vacuum.
The generation of heat is typically effected by means of an electric heater that includes a heating element, and which permits relatively precise setting of the desired temperature. The heating element is commonly implemented by a heating coil. Due to its comparatively high electrical resistance, the coil is typically activated by a current flow, with which electrical energy is converted into thermal energy. The electric current is usually fed selectively to the coil by means of a semiconductor switch, thus avoiding propagation of an electric arc in this region. As a result of the comparatively high current flow and the comparatively high thermal load, damage to the switch or even to the heating element is possible. For example, individual or groups of heating wires or turns of the heating coil may be or become short circuited, such that although a flow of current through the coil remains, the resistance of the heating element is comparatively low. As a result, under such circumstances the thermoplastic synthetic material will no longer be fully heated and the intended desired shaping or deformation of the material will no longer be possible.
In order to avoid damage to the system that is used for deformation of the thermoplastic material, it is therefore important to detect such failures within a relatively short time period after the failure occurs. And, to additionally to ensure that any operational stoppage of the system is comparatively short, it is likewise necessary to quickly identify the relevant fault, i.e. to establish whether it is the switch or the heating element that has failed or is damaged. Toward that end, an electric voltage measuring device that monitors voltage is normally connected in parallel with the switch and the heating element. An unexpectedly marked voltage drop is used as a condition for identifying damage to at least a part of the electric heater.
Preventing damage to the voltage measuring device as a result of an unusually high current flow is achieved by electrically isolating it from the electrical circuit containing the heating element. An optocoupler is typically used to provide such electrical isolation, but optocouplers have only a limited service life. An optocoupler is moreover characterized by relatively rapid aging, and must therefore be replaced at specific time intervals in order to ensure that the electric voltage measuring device continues to function correctly.